In this summary we indicate our uncertainty in observations, mechanisms, and scenarios by using a five-point scale, from "very high confidence" (*****) to "very low confidence" (*).

Climate Changes in the 20th Century

Although there are some regional anomalies, there is strong evidence that climate change has had an impact in the Arctic and the Antarctic. Many documented changes already parallel those forecast to result from climate change:

• In the Arctic, extensive land areas show a 20th-century warming trend in air temperature of as much as 5°C. Over sea ice, there has been slight warming in the 1961-1990 period.***** Precipitation has increased.**
• Arctic sea-ice extent has decreased by 2.9% per decade over the 1978-1996 period; sea ice has thinned, and there are now more melt days per summer. Sea-ice extent in the Nordic seas has decreased by 30% over the past 130 years.***** It is not yet clear whether changes in sea ice of the past few decades are linked to a natural cycle in climate variability or have resulted explicitly from global warming
• Atlantic water flowing into the Arctic Ocean has warmed, and the surface layer has become thinner. The mixed layer in the Beaufort Sea has become less saline.****
• Regions underlain by permafrost have been reduced in extent, and a general warming of ground temperatures has been observed in many areas.*****
• There has been a statistically significant decrease in spring snow extent over Eurasia since 1915.****
• In summary, many observations of environmental change in the Arctic show a trend that is consistent with warming and similar to that predicted by general circulation models (GCMs).
• In the Antarctic, over the past half-century there has been a marked warming trend in the Antarctic Peninsula.**** Elsewhere there is a general but not unambiguous warming trend.**
• Precipitation in the Antarctic has increased.*
• Satellite observations show no significant change in Antarctic sea-ice extent over the 1973-1996 period.***** Analysis of whaling records and modeling studies indicate that Antarctic sea ice retreated south by 2.8 degrees of latitude between the mid-1950s and the early 1970s.***
• Surface waters of the Southern Ocean have warmed and become less saline.***

Impacts

Substantial warming and increases in precipitation are projected for polar regions over the 21st century by almost all climate models. There are eight key concerns related to the impact of this climate change in the Arctic and Antarctic. Associated with these concerns will be changes to the atmosphere and the oceans that will propagate to other regions of the world:

1. Changes in ice sheets and polar glaciers: Increased melting is expected on Arctic glaciers and the Greenland ice sheet, and they will retreat and thin close to their margins. Most of the Antarctic ice sheet is likely to thicken as a result of increased precipitation. There is a small risk, however, that the West Antarctic and Greenland ice sheets will retreat in coming centuries. Together, these cryospheric changes may make a significant contribution to sea-level rise.****
2. Changes around the Antarctic Peninsula: This region has experienced spectacular retreat and collapse of ice shelves, which has been related to a southerly migration of the January 0°C isotherm resulting from regional warming. The loss of these ice shelves has few direct impacts. Projected warming is likely, however, to break up ice shelves further south on the Antarctic Peninsula, expose more bare ground, and cause changes in terrestrial biology, such as introduction of exotic plants and animals.****
3. Changes in the Southern Ocean and impacts on its life: Climate change is likely to produce long-term—perhaps irreversible—changes in the physical oceanography and ecology of the Southern Ocean. Projected reductions in sea-ice extent will alter under-ice biota and spring bloom in the sea-ice marginal zone and will cause profound impacts at all levels in the food chain, from algae to krill to the great whales. Marine mammals and birds, which have life histories that tie them to specific breeding sites, will be severely affected by shifts in their foraging habitats and migration of prey species. Warmer water will potentially intensify biological activity and growth rates of fish. Ultimately, this should lead to an increase in the catch of marketable fish, and retreat of sea ice will provide easier access to southern fisheries.***
4. Changes in sea ice: There will be substantial loss of sea ice in the Arctic Ocean. Predictions for summer ice indicate that its extent could shrink by 60% for a doubling of carbon dioxide (CO₂), opening new sea routes. This will have major trading and strategic implications. With more open water, there will be a moderation of temperatures and an increase in precipitation in Arctic lands. Antarctic sea-ice volume is predicted to decrease by 25% or more for a doubling of CO₂, with sea ice retreating about 2 degrees of latitude.****
5. Changes in permafrost: Thickening of the seasonally thawed layer above permafrost (active layer) is expected. Modeling studies indicate that large areas of permafrost terrain will begin to thaw, leading to changes in drainage, increased mass movements, thermal erosion, and altered landscapes in much of the Arctic and subarctic. Warming of permafrost, thawing of ground ice, and development of thermokarst terrain have been documented over the past several decades. In developed areas of the Arctic, continuation of such changes may lead to costly damage to human infrastructure.****
6. Changes in Arctic hydrology: The hydrology of the Arctic is particularly susceptible to warming because small rises in temperature will result in increased melting of snow and ice, with consequent impacts on the water cycle. There will be a shift to a runoff regime that is driven increasingly by rainfall, with less seasonal variation in runoff. There will be more ponding of water in some areas, but peatlands may dry out because of increased evaporation and transpiration from plants. In some areas, thawing of permafrost will improve infiltration. An expected reduction in ice-jam flooding will have serious impacts on riverbank ecosystems and aquatic ecology, particularly in the highly productive Arctic river deltas. Changes in Arctic runoff will affect sea-ice production, deepwater formation in the North Atlantic, and regional climate. A major impact would result from a weakening of the global thermohaline circulation as a result of a net increase in river flow and the resulting increased flux of freshwater from the Arctic Ocean.***
7. Changes in Arctic biota: Warming should increase biological production; however, the effects of increased precipitation on biological production are unclear. As warming occurs, there will be changes in species compositions on land and in the sea, with a tendency for poleward shifts in species assemblages and loss of some polar species. Changes in sea ice will alter the seasonal distributions, geographic ranges, patterns of migration, nutritional status, reproductive success, and ultimately the abundance and balance of species. Animals that are dependent on sea ice—such as seals, walrus, and polar bears—will be disadvantaged. High-arctic plants will show a strong growth response to summer warming. It is unlikely that elevated CO₂ levels will increase carbon accumulation in plants, but they may be damaged by higher ultraviolet-B radiation. Biological production in lakes and ponds will increase.***
8. Impacts on human communities: Climate change, in combination with other stresses, will affect human communities in the Arctic. The impacts may be particularly disruptive for communities of indigenous peoples following traditional lifestyles. Changes in sea ice, seasonality of snow, and habitat and diversity of food species will affect hunting and gathering practices and could threaten longstanding traditions and ways of life. On the other hand, communities that practice these lifestyles may be sufficiently resilient to cope with these changes. Increased economic costs are expected to affect infrastructure, in response to thawing of permafrost and reduced transportation capabilities across frozen ground and water. However, there will be economic benefits—including new opportunities for trade and shipping across the Arctic Ocean, lower operational costs for the oil and gas industry, lower heating costs, and easier access for ship-based tourism.*****

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